This is a note to let you know that I've just added the patch titled

vt: keyboard, extend func_buf_lock to readers

to my tty git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 82e61c3909db51d91b9d3e2071557b6435018b80 Mon Sep 17 00:00:00 2001

From: Jiri Slaby jslaby@suse.cz Date: Mon, 19 Oct 2020 10:55:17 +0200 Subject: vt: keyboard, extend func_buf_lock to readers

Both read-side users of func_table/func_buf need locking. Without that, one can easily confuse the code by repeatedly setting altering strings like: while (1) for (a = 0; a < 2; a++) { struct kbsentry kbs = {}; strcpy((char *)kbs.kb_string, a ? ".\n" : "88888\n"); ioctl(fd, KDSKBSENT, &kbs); }

When that program runs, one can get unexpected output by holding F1 (note the unxpected period on the last line): . 88888 .8888

So protect all accesses to 'func_table' (and func_buf) by preexisting 'func_buf_lock'.

It is easy in 'k_fn' handler as 'puts_queue' is expected not to sleep. On the other hand, KDGKBSENT needs a local (atomic) copy of the string because copy_to_user can sleep. Use already allocated, but unused 'kbs->kb_string' for that purpose.

Note that the program above needs at least CAP_SYS_TTY_CONFIG.

This depends on the previous patch and on the func_buf_lock lock added in commit 46ca3f735f34 (tty/vt: fix write/write race in ioctl(KDSKBSENT) handler) in 5.2.

Likely fixes CVE-2020-25656.

Cc: stable@vger.kernel.org Reported-by: Minh Yuan yuanmingbuaa@gmail.com Signed-off-by: Jiri Slaby jslaby@suse.cz Link: https://lore.kernel.org/r/20201019085517.10176-2-jslaby@suse.cz Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/vt/keyboard.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c index 7bfa95c3252c..78acc270e39a 100644 --- a/drivers/tty/vt/keyboard.c +++ b/drivers/tty/vt/keyboard.c @@ -743,8 +743,13 @@ static void k_fn(struct vc_data *vc, unsigned char value, char up_flag) return;

if ((unsigned)value < ARRAY_SIZE(func_table)) { + unsigned long flags; + + spin_lock_irqsave(&func_buf_lock, flags); if (func_table[value]) puts_queue(vc, func_table[value]); + spin_unlock_irqrestore(&func_buf_lock, flags); + } else pr_err("k_fn called with value=%d\n", value); } @@ -1991,7 +1996,7 @@ int vt_do_kdsk_ioctl(int cmd, struct kbentry __user *user_kbe, int perm, #undef s #undef v

-/* FIXME: This one needs untangling and locking */ +/* FIXME: This one needs untangling */ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm) { struct kbsentry *kbs; @@ -2023,10 +2028,14 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm) switch (cmd) { case KDGKBSENT: { /* size should have been a struct member */ - unsigned char *from = func_table[i] ? : ""; + ssize_t len = sizeof(user_kdgkb->kb_string); + + spin_lock_irqsave(&func_buf_lock, flags); + len = strlcpy(kbs->kb_string, func_table[i] ? : "", len); + spin_unlock_irqrestore(&func_buf_lock, flags);

- ret = copy_to_user(user_kdgkb->kb_string, from, - strlen(from) + 1) ? -EFAULT : 0; + ret = copy_to_user(user_kdgkb->kb_string, kbs->kb_string, + len + 1) ? -EFAULT : 0;

goto reterr; }